Introduction
If once the milk was bought every day, due to the absence of refrigerators and different living habits, today we tend to do the shopping once every 10-30 days. This market evolution has led to the search for innovative methods to prolong food storage times. Among these, UHT is certainly one of the most used because it is applied to the sterilization of milk and fruit juices.
Sterilized milk, home sterilization
Sterilized is the milk that has undergone a final sterilizing heat treatment in a sealed container. Classical sterilization, known as appertization, consists in bringing the milk, homogenized and bottled, at about 120 ° C for 15-20 minutes and then cooling it by immersion in cold water. This milk has now been replaced by UHT, which is kept less long, but is better from an organoleptic - nutritional point of view and is therefore preferred by the consumer.
To guarantee the healthiness of a food when it is pasteurized or sterilized with heat it is essential to reach the thermal center. If we decide to heat in a small pot of milk opened for a few days we have to face problems that few consider. First of all, due to the coagulation of the whey proteins, the so-called hat is created, an air bubble that protects the micro-organisms from heat. Secondly it is difficult to reach a thermal center at the correct sterilization temperature (while the milk in contact with the walls is very hot, the one in the middle tends to remain colder). To perform a correct operation it is possible to heat the milk with the microwave, which allows to reach high temperatures from the center to the periphery. If this appliance is missing in the kitchen, when you want to pasteurize the milk, bring it to the boil for at least 10-15 minutes, taking care to mix frequently or use a special mixer. It is also important to heat only the quantity that you intend to consume, since repeating this operation several times would also add up considerable nutritional losses.
To heat the milk in the bar, the action of a steam jet at 120 ° C is used, which goes from the center to the periphery; despite the short time of exposure to these temperatures, this technique allows to obtain a milk that is certainly healthier than the one obtained from homemade pasteurization.
UHT milk
UHT stands for Ultra High Temperature. This term refers to a modern process of sterilization of milk which uses very high temperatures for very short periods of time, in order to ensure the sterility of food without compromising its organoleptic and nutritional properties.
The sterilization process involves exposing the milk to temperatures between 135 and 140 ° C in relation to the heating time.
Direct UHT or Uperization, Perforated Milk
The direct UHT or uperization method is the best way to obtain a long-lasting milk. The food is micronized and made to meet with superheated steam at 140-145 ° C. Thanks to the micronization of the milk, intimate contact between microorganisms - spores and superheated steam is guaranteed, thus destroying the entire microbial load. The injection, however, dilutes the milk and a downstream system is therefore necessary to recover the added humidity in the form of steam.
Schematically, milk comes:
- homogenized, preheated to 80 ° C and micronized;
then undergoes a direct injection of steam at 13 bar which brings it to 4 "at 140-150 ° C;
- passes into the decompression chamber in partial vacuum (to facilitate the evaporation of water) and at 75 ° C the steam is returned to the system;
- the milk is then refrigerated and packaged in tetrapak containers.
UHT Indirect
The milk and the heating medium are separated by a wall that constitutes the heat exchange surface. The quality is lower compared to the milk obtained by direct UHT treatment and it is easier to perceive that "cooked" taste due to the slight coagulation of the albumins present in the serum (due to the greater time / temperature product).
Schematically, milk comes:
- preheated and homogenized;
- brought to 108 ° C for 30 seconds (it is in this phase that the organoleptic characteristics are slightly altered - Mailard reaction, denaturation and coagulation of proteins, cameralization of sugars -);
- brought to 140 ° C for 2 "through the passage in a close plate heat exchanger in which steam circulates at 142 ° C;
- cooled to 70 ° C;
- once again cooled to 20 ° C;
- packed in tetrapak containers.
